Effects of elevated CO2 on five plant-aphid interactions

We investigated interactions between five species of phloem-feeding aphids (Homoptera: Aphididae) and their host plants at elevated CO₂; Acyrthosiphon pisum (Harris) on Vicia faba L., Aphis nerii Boyer de Fonscolombe on Asclepias syriaca L., Aphis oenotherae Oestlund on Oenothera biennis L., Aulacorthum solani (Kaltenbach) on Nicotiana sylvestris Speg. & Comes and Myzus persicae (Sulzer) on Solanum dulcamara L. Host plants grown at elevated CO₂ generally had greater biomass, leaf area and C:N ratios than those grown at ambient CO₂, while plants with aphids had lower biomass and leaf area than those without aphids.The responses of aphid populations to elevated CO₂ were species-specific with one species increasing (M. persicae), one decreasing (A. pisum), and the other three being unaffected. CO₂ treatment did not affect the proportion of alate individuals produced. In general, aphid abundance was not significantly related to foliar nitrogen concentration.We performed separate analyses to test whether either aphid presence or aphid abundance modified the response of host plants to elevated CO₂. In terms of aphid presence, only three of the potential 15 interactions (five aphid species x three plant traits) were significant; A. solani slightly modified the response of the plant biomass to elevated CO₂ and M. persicae affected the response of leaf area and allocation. In terms of aphid abundance, only two of the potential 15 interactions were significant with A. nerii modifying the plant response to CO₂ in terms of total leaf area and allocation.We conclude that, in contrast to other insect groups such as leaf chewers, populations of most phloem-feeders may not be negatively affected by increased CO₂ concentrations in the future. The reasons for this difference include the possibility that aphids may be able to compensate for changes in host plant quality by altering feeding behaviour or by synthesizing amino acids. In addition, there is little evidence that aphid herbivory, even at high levels, will substantially modify the response of plants to elevated CO₂.     

Impact of elevated CO2 on the third trophic level: A predator Harmonia axyridis and a parasitoid Aphidius picipes

The effects of elevated CO₂ (550 and 750 µL/L vs. ambient CO₂) on the third trophic level, a predator Harmonia axyridis (Pallas) and a parasitoid Aphidius picipes (Nees), were studied in open-top chambers. The impact of elevated CO₂ on the growth and development of H. axyridis was either weak or nonexistent, whereas the abundance of the parasitized aphid (Sitobion avenae Fabricius) by A. picipes showed a significant increase in 550 (12.5%) and 750 (19.6%) µL/L CO₂ compared to ambient CO₂, respectively. In addition, there was a significant decrease (10%) in the emergence rate of A. picipes under 750 µL/L CO₂ compared to ambient CO₂ (P<0.05). The predator and the parasitoid both substantially suppressed aphid abundance, especially in elevated CO₂ for A. picipes. Moreover, H. axyridis and A. picipes preferred to prey on/parasitize more aphids, S. avenae, infested on 550 (9.1 and 16.9%) and 750 (23 and 25.7%) µL/L CO₂-grown wheat plants than those fed ambient CO₂-grown wheat plants. These initial results indicate that elevated CO₂ markedly changes the predation/parasitization preference by the predator/parasitoid for wheat aphids. The biocontrol efficiency of A. picipes against S. avenae can be enhanced in elevated CO₂; simultaneously, elevated CO₂ has adverse effects on the growth and development of A. picipes-parasitized S. avenae.     

Aphid response to elevated ozone and CO2

Numerous reports indicate that pollution stress caused by sulphur dioxide (SO₂), oxies of nitrogen or fluorides promote aphid growth on herbaceous and woody plants. At SO₂ exposures, the response curve of aphids is bell-shaped having the peak at 100 ppb. This curvilinear response is related to physiological stress responses of host plants exposed to pollutants. On the other hand, observations of aphid performance on ozone-exposed (O₃) or elevated carbon dioxide-exposed (CO₂) plants have given very variable results. Depending on the duration and concentration of O₃ or elevated CO₂ exposure or the age of the exposed plants, aphid growth on the same plants either decreased or increased in comparison to growth on control plants grown in filtered air. The results of these studies suggest that there is no general air pollution-induced plant stress that triggers aphid outbreaks on plants. Plants grown in elevated CO₂ usually have higher C/N ratios than plants grown in current ambient CO₂ atmosphere. A reduced proportion of nitrogen in the plant foliage decreases growth of chewing herbivorous insects, but the few studies of elevated CO₂ effects on sucking insects such as aphids have not yielded similar consistent effects. The present paper reviews recent studies of elevated CO₂ effects on aphids and discusses the effects of combined elevated O₃ and CO₂ exposures on aphid performance on woody plants using pine and birch aphids as examples.     

Host-specific aphid population responses to elevated CO2 and increased N availability

Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO₂. Recent models predict that increased nitrogen will increase aphid population size under elevated CO₂, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO₂, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C₃ plant (Solanum dulcamara), and a C₄ plant (Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO₂, with the degree of increase depending on the N availability. We found a significant CO₂× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO₂, low N conditions increased in response to either high N availability or elevated CO₂. No population size responses were observed for aphids infesting A. viridis. Elevated CO₂ increased plant biomass, specific leaf weight, and C : N ratios of the C₃ plant, S. dulcamara but did not affect the C₄ plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO₂ decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO₂ or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity.     

Pea aphid promotes amino acid metabolism both in Medicago truncatula and bacteriocytes to favor aphid population growth under elevated CO2

Rising atmospheric CO₂ levels can dilute the nitrogen (N) resource in plant tissue, which is disadvantageous to many herbivorous insects. Aphids appear to be an exception that warrants further study. The effects of elevated CO₂ (750 ppm vs. 390 ppm) were evaluated on N assimilation and transamination by two Medicago truncatula genotypes, a N‐fixing‐deficient mutant (dnf1) and its wild‐type control (Jemalong), with and without pea aphid (Acyrthosiphon pisum) infestation. Elevated CO₂ increased population abundance and feeding efficiency of aphids fed on Jemalong, but reduced those on dnf1. Without aphid infestation, elevated CO₂ increased photosynthetic rate, chlorophyll content, nodule number, biomass, and pod number for Jemalong, but only increased pod number and chlorophyll content for dnf1. Furthermore, aphid infested Jemalong plants had enhanced activities of N assimilation‐related enzymes (glutamine synthetase, Glutamate synthase) and transamination‐related enzymes (glutamate oxalate transaminase, glutamine phenylpyruvate transaminase), which presumably increased amino acid concentration in leaves and phloem sap under elevated CO₂. In contrast, aphid infested dnf1 plants had decreased activities of N assimilation‐related enzymes and transmination‐related enzymes and amino acid concentrations under elevated CO₂. Furthermore, elevated CO₂ up‐regulated expression of genes relevant to amino acid metabolism in bacteriocytes of aphids associated with Jemalong, but down‐regulated those associated with dnf1. Our results suggest that pea aphids actively elicit host responses that promote amino acid metabolism in both the host plant and in its bacteriocytes to favor the population growth of the aphid under elevated CO₂.     

Aphid and host‐plant genotype × genotype interactions under elevated CO2

1. Elevated CO₂ can alter plant physiology and morphology, and these changes are expected to impact diet quality for insect herbivores. While the plastic responses of insect herbivores have been well studied, less is known about the propensity of insects to adapt to such changes. Genetic variation in insect responses to elevated CO₂ and genetic interactions between insects and their host plants may exist and provide the necessary raw material for adaptation. 2. We used clonal lines of Rhopalosiphum padi (L.) aphids to examine genotype‐specific responses to elevated CO₂. We used the host plant Schedonorus arundinaceus (tall fescue; Schreb), which is capable of asexual reproduction, to investigate host plant genotype‐specific effects and possible host plant‐by‐insect genotype interactions. The abundance and density of three R. padi genotypes on three tall fescue genotypes under three concentrations of CO₂ (ambient, 700, and 1000 ppm) in a controlled greenhouse environment were examined. 3. Aphid abundance decreased in the 700 ppm CO₂ concentration, but increased in the 1000 ppm concentration relative to ambient. The effect of CO₂ on aphid density was dependent on host plant genotype; the density of aphids in high CO₂ decreased for two plant genotypes but was unchanged in one. No interaction between aphid genotype and elevated CO₂ was found, nor did we find significant genotype‐by‐genotype interactions. 4. This study suggests that the density of R. padi aphids feeding on tall fescue may decrease under elevated CO₂ for some plant genotypes. The likely impact of genotype‐specific responses on future changes in the genetic structure of plant and insect populations is discussed.     

Aphid–hoverfly interactions under elevated CO2 concentrations: oviposition and larval development

The performance of predators of plant pests is mainly driven by their ability to find prey. Recent studies suggest that rising atmospheric carbon dioxide (CO₂) concentrations will affect the semiochemistry of plant–insect relationships, possibly altering prey‐finding behaviour. In the present study, we test the hypothesis that higher atmospheric CO₂ concentrations affect the oviposition behaviour of an aphidophagous hoverfly and alter the development of its larvae. We also test the hypothesis that volatile compounds released by the plant–aphid association are modified under elevated CO_2. Broad bean plants infested with pea aphids are grown under ambient (450 ppm) or elevated CO₂ (800 ppm) concentrations. Plants raised under each treatment are then presented to gravid hoverfly females in a dual‐choice bioassay. In addition, emerging Episyrphus balteatus larvae are directly fed with aphids reared under ambient or elevated CO₂ conditions and then measured and weighed daily until pupation. Odours emitted by the plant–aphid association are sampled. A larger number of eggs is laid on plants grown under ambient CO₂ conditions. However, no significant difference is observed between the two groups of predatory larvae grown under different CO₂ concentrations, indicating that the CO₂ concentration does not affect the quality of their aphid diet. Although plant volatiles do not differ between the ambient and elevated CO₂‐treated plants, we find that the quantity of aphid alarm pheromone is lower on the plant–aphid association raised under the elevated CO₂ condition. This suggests that an alteration of semiochemical emissions by elevated CO₂ concentrations impacts the oviposition behaviour of aphid predators.     

Towards an understanding of how phloem amino acid composition shapes elevated CO2‐induced changes in aphid population dynamics

1. The performance of foliage feeders tends to decrease under elevated CO₂, but the responses of phloem‐feeding insects have been much more equivocal. As phloem tissues are less accessible than whole‐plant tissues, much less is known about how phloem composition is altered under elevated CO₂ and the mechanisms driving changes in aphid performance. 2. In this study, the plant mechanisms underlying the performance of Rhopalosiphum padi aphids on Hordeum vulgare (barley) grown under ambient (390 ppm) and elevated (700 ppm) CO₂ were examined. We used aphid stylectomy to sample pure phloem from plants in CO₂‐controlled conditions and high‐performance liquid chromatography to analyse phloem samples for amino acid concentrations. 3. Aphid abundance significantly increased by 127% under elevated CO₂. Consequently, plant biomass decreased under elevated CO₂ in trials with herbivores present, possibly due to the increased herbivore load, but increased when aphids were absent. The intrinsic rate of population increase (r_m) was significantly higher under elevated CO₂; however, there were no statistically significant effects on aphid fecundity or development time. The concentration of individual amino acids tended to increase, although these increases were statistically significant in only a few cases. A principal components analysis revealed that the relative abundance (mol %) of those amino acids considered essential for aphids tended to increase under elevated CO₂. 4. These results indicate that CO₂ may affect nutrient translocation in plants in ways that are contrary to predictions about nitrogen metabolite responses to CO₂. Such plant biochemical responses may underlie observations of improved phloem feeder performance under elevated CO₂.     

Effects of carbon dioxide enrichment on response of mycorrhizal pitch pine (Pinus rigida) to aluminum: growth and mineral nutrition

 Carbon dioxide enrichment may increase the Al tolerance of trees by increasing root growth, root exudation and/or mycorrhizal colonization. The effect of elevated CO₂ on the response of mycorrhizal pitch pine (Pinus rigida Mill.) seedlings to Al was determined in two experiments with different levels of nutrients, 0.1- or 0.2-strength Clark solution. During each experiment, seedlings inoculated with the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown 13 weeks in sand irrigated with nutrient solution (pH 3.8) containing 0, 6.25, 12.5, or 25 mg/l Al (0, 232, 463, or 927 μM Al) in growth chambers fumigated with 350 (ambient) or 700 (elevated) μl/l CO₂. At ambient CO₂, in the absence of Al, mean total dry weights (DW) of seedlings at the high nutrient level were 164% higher than those at the low level. Total DW at elevated CO₂, in the absence of Al, was significantly greater than that in ambient CO₂ at the low (+34%) and high (+16%) nutrient levels. Root and shoot DW at both nutrient levels decreased with increasing Al concentrations with Al reducing root growth more than shoot growth. Although visible symptoms of Al toxicity in roots and needles were reduced by CO₂ enrichment, there were no significant CO₂ × Al interactions for shoot or root DW. The percentage of seedling roots that became mycorrhizal was negatively related to nutrient level and was greater at elevated than at ambient CO₂ levels. Generally, elevated CO₂ had little effect on concentration of mineral nutrients in roots and needles. Aluminum reduced concentrations of most nutrients by inhibiting uptake. Received: 18 June 1997 / Accepted: 8 December 1997     

Climate change and cereal aphids: the relative effects of increasing CO2 and temperature on aphid population dynamics

Experimental evidence regarding the responses of cereal aphids to rising atmospheric CO₂ has been ambiguous. Some studies suggest increased population sizes under future CO₂ levels, others suggest decreased population sizes, and still others suggest little or no difference. Recently, Newman et al. (2003) constructed a general mathematical model of the aphid–grass interaction to investigate whether or not we should, in fact, expect a general aphid response to rising CO₂. They concluded that aphid populations are likely to be larger under future CO₂ concentrations if soil N levels are high, the aphid species' nitrogen requirement is low and the aphid species' density‐dependent response in winged morph production is weak. In that model, and in field experiments, CO₂ concentration influences aphid population dynamics through the effect it has on plant quality. However, future CO₂ concentrations are also likely to be accompanied by higher ambient temperatures, a combination that has received little focus to date. In the present paper, the Newman et al. model is used to consider the combined effects of increased CO₂ concentrations and temperature on aphid population sizes. It is concluded that, when both factors are elevated, aphid population dynamics will be more similar to current ambient conditions than expected from the results of experiments studying either factor alone. This result has important implications for future experimentation.     

Performance and allocation patterns of the perennial herb,Plantago lanceolata,in response to simulated herbivory and elevated CO2 environments

Summary We tested the prediction that plants grown in elevated CO₂ environments are better able to compensate for biomass lost to herbivory than plants grown in ambient CO₂ environments. The herbaceous perennial Plantago lanceolata (Plantaginaceae) was grown in either near ambient (380 ppm) or enriched (700 ppm) CO₂ atmospheres, and then after 4 weeks, plants experienced either 1) no defoliation; 2) every fourth leaf removed by cutting; or 3) every other leaf removed by cutting. Plants were harvested at week 13 (9 weeks after simulated herbivory treatments). Vegetative and reproductive weights were compared, and seeds were counted, weighed, and germinated to assess viability.Plants grown in enriched CO₂ environments had significantly greater shoot weights, leaf areas, and root weights, yet had significantly lower reproductive weights (i.e. stalks + spikes + seeds) and produced fewer seeds, than plants grown in ambient CO₂ environments. Relative biomass allocation patterns further illustrated differences in plants grown in ambient CO₂ environments. Relative biomass allocation patterns further illustrated differences in plant responses to enriched CO₂ atmospheres: enriched CO₂-grown plants only allocated 10% of their carbon resources to reproduction whereas ambient CO₂-grown plants allocated over 20%. Effects of simulated herbivory on plant performance were much less dramatic than those induced by enriched CO₂ atmospheres. Leaf area removal did not reduce shoot weights or reproductive weights of plants in either CO₂ treatment relative to control plants. However, plants from both CO₂ treatments experienced reductions in root weights with leaf area removal, indicating that plants compensated for lost above-ground tissues, and maintained comparable levels of reproductive output and seed viability, at the expense of root growth.     

Elevated CO2 alters the feeding behaviour of the pea aphid by modifying the physical and chemical resistance of Medicago truncatula

Elevated CO₂ compromises the resistance of leguminous plants against chewing insects, but little is known about whether elevated CO₂ modifies the resistance against phloem‐sucking insects or whether it has contrasting effects on the resistance of legumes that differ in biological nitrogen fixation. We tested the hypothesis that the physical and chemical resistance against aphids would be increased in Jemalong (a wild type of Medicago truncatula) but would be decreased in dnf1 (a mutant without biological nitrogen fixation) by elevated CO₂. The non‐glandular and glandular trichome density of Jemalong plants increased under elevated CO₂, resulting in prolonged aphid probing. In contrast, dnf1 plants tended to decrease foliar trichome density under elevated CO₂, resulting in less surface and epidermal resistance to aphids. Elevated CO₂ enhanced the ineffective salicylic acid‐dependent defence pathway but decreased the effective jasmonic acid/ethylene‐dependent defence pathway in aphid‐infested Jemalong plants. Therefore, aphid probing time decreased and the duration of phloem sap ingestion increased on Jemalong under elevated CO₂, which, in turn, increased aphid growth rate. Overall, our results suggest that elevated CO₂ decreases the chemical resistance of wild‐type M. truncatula against aphids, and that the host's biological nitrogen fixation ability is central to this effect.     

Plant stomatal closure improves aphid feeding under elevated CO2

Stomata help plants regulate CO₂ absorption and water vapor release in response to various environmental changes, and plants decrease their stomatal apertures and enhance their water status under elevated CO₂. Although the bottom‐up effect of elevated CO₂ on insect performance has been extensively studied, few reports have considered how insect fitness is altered by elevated CO₂‐induced changes in host plant water status. We tested the hypothesis that aphids induce stomatal closure and increase host water potential, which facilitates their passive feeding, and that this induction can be enhanced by elevated CO₂. Our results showed that aphid infestation triggered the abscisic acid (ABA) signaling pathway to decrease the stomatal apertures of Medicago truncatula, which consequently decreased leaf transpiration and helped maintain leaf water potential. These effects increased xylem‐feeding time and decreased hemolymph osmolarity, which thereby enhanced phloem‐feeding time and increased aphid abundance. Furthermore, elevated CO₂ up‐regulated an ABA‐independent enzyme, carbonic anhydrase, which led to further decrease in stomatal aperture for aphid‐infested plants. Thus, the effects of elevated CO₂ and aphid infestation on stomatal closure synergistically improved the water status of the host plant. The results indicate that aphid infestation enhances aphid feeding under ambient CO₂ and that this enhancement is increased under elevated CO₂.     

Response of amino acid changes in Aphis gossypii (Glover) to elevated CO2 levels

Effects of elevated CO₂ levels on the amino acid constituents of cotton aphid, Aphis gossypii (Glover), fed on transgenic Bacillus thuringiensis (Berliner) (Bt) cotton [Cryl A(c)], grown in ambient and double‐ambient CO₂ levels in closed‐dynamics CO₂ chambers, were investigated. Lower amounts of amino acids were found in cotton phloem under elevated CO₂ than under ambient CO₂ levels. However, higher amounts of free amino acids were found in A. gossypii fed on elevated CO₂‐grown cotton than those fed ambient CO₂‐grown cotton, and the contents of amino acids in honeydew were not significantly affected by elevated CO₂ levels. A larger amount of honeydew was produced by cotton aphids feeding on leaves under elevated CO₂ treatment than those feeding on leaves under ambient CO₂ treatment, which indicates that A. gossypii ingests more cotton phloem because of the higher C:N ratio of cotton phloem under elevated CO₂ levels. Moreover, the amino acid composition was similar in bodies of aphids ingesting leaves under both CO₂ treatments, except for two alkaline amino acids, lysine and arginine. This suggests that the nutritional constitution of the phloem sap was important for A. gossypii. Our data suggest that more phloem sap will be ingested by A. gossypii to satisfy its nutritional requirement and balance the break‐even point of amino acid in elevated CO₂. Larger amounts of honeydew produced by A. gossypii under elevated CO₂ will reduce the photosynthesis and result in the occurrence of some Entomophthora spp.     

Effects of elevated CO2 and plant genotype on interactions among cotton,aphids and parasitoids

Abstract Effects of CO₂ level (ambient vs. elevated) on the interactions among three cotton (Gossypium hirsutum) genotypes, the cotton aphid (Aphis gossypii Glover), and its hymenoptera parasitoid (Lysiphlebia japonica Ashrnead) were quantified. It was hypothesized that aphid‐parasitoid interactions in crop systems may be altered by elevated CO₂, and that the degree of change is influenced by plant genotype. The cotton genotypes had high (M9101), medium (HZ401) and low (ZMS13) gossypol contents, and the response to elevated CO₂ was genotype‐specific. Elevated CO₂ increased the ratio of total non‐structural carbohydrates to nitrogen (TNC: N) in the high‐gossypol genotype and the medium‐gossypol genotype. For all three genotypes, elevated CO₂ had no effect on concentrations of gossypol and condensed tannins. A. gossypii fitness declined when aphids were reared on the high‐gossypol genotype versus the low‐gossypol genotype under elevated CO₂. Furthermore, elevated CO₂ decreased the developmental time of L. japonica associated with the high‐gossypol genotype and the low‐gossypol genotype, but did not affect parasitism or emergence rates. Our study suggests that the abundance of A. gossypii on cotton will not be directly affected by increases in atmospheric CO₂. We speculate that A. gossypii may diminish in pest status in elevated CO₂ and high‐gossypol genotype environments because of reduced fitness to the high‐gossypol genotype and shorter developmental time of L. japonica.     

Interactions of elevated carbon dioxide and temperature with aphid feeding on transgenic oilseed rape: Are Bacillus thuringiensis (Bt) plants more susceptible to nontarget herbivores in future climate?

Climate change factors such as elevated carbon dioxide (CO₂) and temperature typically affect carbon (C) and nitrogen (N) dynamics of crop plants and the performance of insect herbivores. Insect‐resistant transgenic plants invest some nutrients to the production of specific toxic proteins [i.e. endotoxins from Bacillus thuringiensis (Bt)], which could alter the C–N balance of these plants, especially under changed abiotic conditions. Aphids are nonsusceptible to Lepidoptera‐targeted Bt Cry1Ac toxin and they typically show response to abiotic conditions, and here we sought to discover whether they might perform differently on compositionally changed Bt oilseed rape. Bt oilseed rape had increased N content in the leaves coupled with reduced total C compared with its nontransgenic counterpart, but in general the C : N responses of both plant types to elevated CO₂ and temperature were similar. Elevated CO₂ decreased N content and increased C : N ratio of both plant types. Elevated temperature increased C and N contents, total chlorophyll and carotenoid concentrations under ambient CO₂, but decreased these under elevated CO₂. In addition, soluble sugars were increased and starch decreased by elevated temperature under ambient but not under elevated CO₂, whereas photosynthesis was decreased in plants grown under elevated temperature in both CO₂ levels. Myzus persicae, a generalist aphid species, responded directly to elevated temperature with reduced developmental time and decreased adult and progeny weights, whereas the development of the Brassica specialist Brevicoryne brassicae was less affected. Feeding by M. persicae resulted in an increase in the N content of oilseed rape leaves under ambient CO₂, indicating the potential of herbivore feeding itself to cause allocation changes. The aphids performed equally well on both plant types despite the differences between C–N ratios of Bt and non‐Bt oilseed rape, revealing the absence of plant composition‐related effects on these pests under elevated CO₂, elevated temperature or combined elevated CO₂ and temperature conditions.     

Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris

Among plants grown under enriched atmospheric CO₂, root:shoot balance (RSB) theory predicts a proportionately greater allocation of assimilate to roots than among ambient‐grown plants. Conversely, defoliation, which decreases the plant's capacity to assimilate carbon, is predicted to increase allocation to shoot. We tested these RSB predictions, and whether responses to CO₂ enrichment were modified by defoliation, using Heterotheca subaxillaris, an annual plant native to south‐eastern USA. Plants were grown under near‐ambient (400 μmol mol^?1) and enriched (700 μmol mol^?1) levels of atmospheric CO₂. Defoliation consisted of the weekly removal of 25% of each new fully expanded, but not previously defoliated, leaf from either rosette or bolted plants. In addition to dry mass measurements of leaves, stems, and roots, Kjeldahl N, protein, starch and soluble sugars were analysed in these plant components to test the hypothesis that changes in C:N uptake ratio drive shifts in root:shoot ratio. Young, rapidly growing CO₂‐enriched plants conformed to the predictions of RSB, with higher root:shoot ratio than ambient‐grown plants (P < 0.02), whereas older, slower growing plants did not show a CO₂ effect on root:shoot ratio. Defoliation resulted in smaller plants, among which both root and shoot biomass were reduced, irrespective of CO₂ treatment (P < 0.03). However, H. subaxillaris plants were able to compensate for leaf area removal through flexible shoot allocation to more leaves vs. stem (P < 0.01). Increased carbon availability through CO₂ enrichment did not enhance the response to defoliation, apparently because of complete growth compensation for defoliation, even under ambient conditions. CO₂‐enriched plants had higher rates of photosynthesis (P < 0.0001), but this did not translate into increased final biomass accumulation. On the other hand, earlier and more abundant yield of flower biomass was an important consequence of growth under CO₂ enrichment.     

Effects of elevated CO2 and/or O3 on growth, development and physiology of wheat (Triticum aestivum L.)

Two cultivars of spring wheat (Triticum aestivum L. cvs. Alexandria and Hanno) and three cultivars of winter wheat (cvs. Riband, Mercia and Haven) were grown at two concentrations of CO₂ [ambient (355 pmol mol^?1) and elevated (708 μmol mol^?1)] under two O₃ regimes [clean air (< 5 nmol mol^?1 O₃) and polluted air (15 nmol mol^?1 O₃ at night rising to a midday maximum of 75 nmol mol^?1)] in a phytotron at the University of Newcastle-upon-Tyne. Between the two-leaf stage and anthesis, measurements of leaf gas-exchange, non-structural carbohydrate content, visible O₃ damage, growth, dry matter partitioning, yield components and root development were made in order to examine responses to elevated CO₂ and/or O₃. Growth at elevated CO₂ resulted in a sustained increase in the rate of CO₂ assimilation, but after roughly 6 weeks' exposure there was evidence of a slight decline in the photosynthetic rate (c.-15%) measured under growth conditions which was most pronounced in the winter cultivars. Enhanced rates of CO₂ assimilation were accompanied by a decrease in stomatal conductance which improved the instantaneous water use efficiency of individual leaves. CO₂ enrichment stimulated shoot and root growth to an equivalent extent, and increased tillering and yield components, however, non-structural carbohydrates still accumulated in source leaves. In contrast, long-term exposure to O₃ resulted in a decreased CO₂ assimilation rate (c. -13%), partial stomatal closure, and the accumulation of fructan and starch in leaves in the light. These effects were manifested in decreased rates of shoot and root growth, with root growth more severely affected than shoot growth. In the combined treatment growth of O₃-treated plants was enhanced by elevated CO₂, but there was little evidence that CO₂ enrichment afforded additional protection against O₃ damage. The reduction in growth induced by O₃ at elevated CO₂ was similar to that induced by O₃ at ambient CO₂ despite additive effects of the individual gases on stomatal conductance that would be expected to reduce the O₃ flux by 20%, and also CO₂-induced increases in the provision of substrates for detoxification and repair processes. These observations suggest that CO₂ enrichment may render plants more susceptible to O₃ damage at the cellular level. Possible mechanisms are discussed.     

Individual growth rates do not predict aphid population densities under altered atmospheric conditions

• 1 Altered atmospheric composition, associated with climate change, can modify herbivore population dynamics through CO₂ and/or O₃‐mediated changes in plant quality.
• 2 Although pea aphid Acyrthosiphon pisum genotypes exhibit intraspecific variation in population growth in response to atmospheric composition, the proximate mechanisms underlying this variation are largely unknown.
• 3 By rearing single (green, pink) and mixed (green + pink) pea aphid genotypes on red clover Trifolium pratense at the Aspen Free Air CO₂ and O₃ Enrichment (Aspen FACE) site, we assessed whether: (i) elevated CO₂ and/or O₃ concentrations alter aphid growth and development and (ii) individual aphid growth rates predict aphid population densities.
• 4 We showed that growth and development of individual green and pink aphids were not influenced by CO₂ and/or O₃ concentrations when reared as individual or mixed genotypes. Individual growth rates, however, did not predict population densities.
• 5 Reared as a single genotype, green pea aphid populations decreased in response to elevated CO₂ concentrations, but not in response to elevated CO₂ + O₃ concentrations. Pink pea aphid populations reared as a single genotype were unaffected by augmented CO₂ or O₃. Populations of mixed genotypes, however, were reduced under elevated CO₂ concentrations, irrespective of O₃ concentrations.
• 6 Herbivore population sizes may not readily be predicted from growth rates of individual organisms under atmospheric conditions associated with global climate change.

     

Elevated CO2 shifts the focus of tobacco plant defences from cucumber mosaic virus to the green peach aphid

Elevation in CO₂ concentration broadly impacts plant physiological characteristics, which influences herbivores and biotrophic pathogens, which in turn regulate the plant defensive response. In this study, responses of tobacco plants to stress in the form of the green peach aphid, Myzus persicae (Sulzer), or cucumber mosaic virus (CMV), or both aphid and CMV combined were investigated in open‐top chambers under ambient and elevated CO₂ concentrations. We measured aboveground biomass and foliar chlorophyll, nitrogen, non‐structural carbohydrates, soluble protein, total amino acid and nicotine content in tobacco plants and also measured aphid population dynamics, body weight, honeydew production and anti‐oxidative enzyme activities in individual aphids. Plants produced more secondary metabolites for defence in both CO₂ treatments when treated with aphid and CMV combined than with either alone. Aphid density significantly increased on CMV‐infected tobacco plants (relative to uninfected plants) under ambient CO₂ but not under elevated CO₂. This suggests that plant defences against virus and aphid would be more efficient under elevated CO₂. Plant defence appears to shift from plant virus to aphid under increasing CO₂ levels, which highlights the potential influences of multiple biotic stressors on plants under elevated CO₂.